Two-stage formation model and helicity of gold nanowires

Electronic transport properties of ultra-thin Ni and Ni-C nanowires

The structures and electronic transport properties of ultra-thin Ni and Ni-C nanowires obtained from carbon nanotube (CNT) templates are theoretically investigated. C atoms tend to locate at the central positions of nanowires and are surrounded by Ni atoms. Spin polarization at the Fermi level is not responsible for the spin filtration of these nanowires. Increasing C concentration can improve the resistance of nanowires by abating the number of electronic transmission channels and the coupling of electron orbitals between Ni atoms. Moreover, with the increase of diameter, the conductance of these nanowires increases as well. This study is helpful for guiding the synthesis of nanowires with desired applications.

Parametrization scheme with accuracy and transferability for tight-binding electronic structure calculations with extended Hückel approximation and molecular dynamics simulations

A transferable tight-binding parametrization procedure for extended Hückel approximation is proposed, with the charge self-consistent scheme, that could be applied to the quantum molecular dynamics (MD) simulation for long-time dynamics of large-scale systems. In this procedure, either a target molecule is divided into small molecules or another realistic set of small molecules characterizing chemical bonds in the complicated target molecule is adopted. Then, the parameters for these small molecules are adjusted and compared with reference results of energy levels and wave functions by, for example, density functional theory. Upon application to the large target molecule, these parameters are then readjusted directly in the target molecule. An example is demonstrated with MD simulation applied to the ionic liquid molecule N-methyl-N-propylpiperidinium bis trifluoromethanesulfonyl imide (PP13-TFSI). The origin and stability of HOMO-LUMO gap are discussed.

An order-N electronic structure theory with generalized eigenvalue equations and its application to a ten-million-atom system

A linear algebraic theory called the 'multiple Arnoldi method' is presented and realizes large-scale (order-N) electronic structure calculations with generalized eigenvalue equations. A set of linear equations, in the form of (zS - H)x = b, are solved simultaneously with multiple Krylov subspaces. The method is implemented in a simulation package ELSES (www.elses.jp) with tight-binding-form Hamiltonians. A finite-temperature molecular dynamics simulation is carried out for metallic and insulating materials. A calculation with 10(7) atoms was realized by a workstation. The parallel efficiency is shown up to 1024 CPU cores.

Coupled effects of size and uniaxial force on phase transitions in copper nanowires

By means of molecular dynamics simulations, we investigate solid-solid phase transitions between the regular crystalline structure and the unbuckled atomic-sized one in ultrathin copper nanowires under uniaxial elongation and compression. The interatomic potential employed has been verified to be capable of describing the structural and mechanical properties of ultrathin copper nanowires. The coupled effects of size, uniaxial force and relaxing temperature on the transitions have been revealed. Both the reported phase transition from the crystalline structure to the helical one and the unexpected inverse behavior are found. At a relaxing temperature of 900 K, helical structures are dominant for effective diameters less than 0.8 nm, while the uniaxial force may lead to helical-crystalline phase transitions for thicker ones. We also observe that the transition from a helical 12-7-1 nanowire to a crystalline [110] nanowire is much easier than the inverse transition. Our results demonstrate the structural phase transitions between the crystalline structures and the unbuckled atomic-sized ones of the force-suspended metal nanowires.

Domain boundary formation in helical multishell gold nanowires

Helical multishell gold nanowires are studied theoretically for the formation mechanism of the helical domain boundary. Nanowires with a wire length of more than 10 nm are relaxed by quantum mechanical molecular dynamics simulation with a tight-binding form Hamiltonian. In the results, non-helical nanowires are transformed into helical ones with the formation of atom pair defects at the domain boundary, where the defective atom pair is moved from an inner shell. Analysis of local electronic structure shows a competitive feature of the energy gain of reconstruction on the wire surface and the energy loss of the defect formation. A simple energy scaling theory gives a general explanation of domain boundary formation.

Development of the simulation package 'ELSES' for extra-large-scale electronic structure calculation

An early-stage version of the simulation package 'ELSES' (extra-large-scale electronic structure calculation) is developed for simulating the electronic structure and dynamics of large systems, particularly nanometer-scale and ten-nanometer-scale systems (see www.elses.jp). Input and output files are written in the extensible markup language (XML) style for general users. Related pre-/post-simulation tools are also available. A practical workflow and an example are described. A test calculation for the GaAs bulk system is shown, to demonstrate that the present code can handle systems with more than one atom species. Several future aspects are also discussed.

Helical [110] gold nanowires make longer linear atomic chains

Quantum mechanical molecular dynamics shows that gold nanowires formed along the [110] direction reconstruct upon stress to form helical nanowires. The mechanism for this formation is discussed. These helical nanowires evolve on stretching to form linear atomic chains. Because helical nanowires do not form symmetrical tips, a requirement to stop the growth of atomic chains, these nanowires produce longer atomic chains than other nanowires. These results are obtained resorting to the use of tight-binding molecular dynamics and ab initio electronic structure calculations.